The present invention relates generally to thin-film magnets, or magnetic thin films, and more specifically relates to the composition of magnetic thin films produced from the reaction of a vanadium containing compound, tetracyanoethylene and a low-temperature chemical vapor deposition processes for depositing a vanadium containing thin-film upon a wide variety of supporting substrates.
Thin films having certain magnetic, electrical, optical properties, low temperature processibility, insulating to semiconducting behavior, mechanical flexibility, solubility, and substrate compatibility properties, as well as other physical properties, are of interest and often times are essential in many applications. Of particular interest are magnetic thin films that may be deposited upon substrates comprised of materials such as plastics, glasses, metals, and ionic crystalline materials and in which the substrates are either rigid or flexible. For example, thin magnetic films are used extensively on magnetic tapes and media used in the storage and retrieval of computer generated data, magnetic shielding, as well as for providing extraction of magnetic substances.
The use of chemical vapor deposition (CVD) techniques for applying various thin films onto a variety of substrates is generally well known. For example, CVD is used extensively in the semiconductor industry to apply thin films having various electrical and magnetic characteristics onto substrates constructed primarily or entirely of semiconductive material containing silicon, germanium, or gallium arsenide and other generally inorganic substrates. Normally CVD techniques used in the semiconductor industry are conducted at temperatures above 200xc2x0 C.
Many advances in the art of producing magnetic materials, including molecule-based magnetic materials have occurred in the past decade. An example of synthesizing such a molecule-based magnetic material, or magnet, includes reacting tetracyanoethylene (TCNE) in a solution of bis(benzene)vanadium, V(C6H6)2, or vanadium hexacarbonyl V(CO)6 in the presence of an aprotic poorly coordinating solvent such as acetonitrile, C2H3N, tetrahydrofiran, C4H8O or dichloromethane, CH2Cl2, at or below room temperature. Such priorly known solvent-based reaction can be symbolically stated: V[TCNE]xxc2x7S wherein Tc isxcx9c400 K; xxcx9c2; yxcx9c0.5; and S≈solvent as set forth in: Manriquez, J. M.; Yee, G. T.; McLean, R. S.; Epstein, A. J.; Miller, J. S., Science 1991, 252, 1415; and Miller, J. S.; Yee, G. T.; Manriquez, J. M.; Epstein, A. J., in the Proceedings of Nobel Symposium #NS-81 Conjugated Polymers and Related Materials: The Interconnection of Chemical and Electronic Structure, Oxford University Press, 1993, 461 with respect to reacting bis(benzene)vanadium, V(C6H6)2, in a solvent and as set forth in Zhang, J.; Zhou, P.; Brinckerhoff, W. B.; Epstein, A. J.; Vazquez, C.; McLean, R. S.; Miller, J. S., A. C. S. Sym. Ser. 1996, 644, 311 with respect to reacting vanadium hexacarbonyl V(CO)6 in a solvent. Such molecule based magnets, or magnetic thin films, synthesized in the presence of a solvent are extremely air sensitive, do not magnetically saturate at lower temperatures, and tend to have an inhomogeneous compositions and non-uniform densities and poorly reproducible magnetic properties. Moreover, molecule based magnets, or magnetic thin films, produced in the presence of a solvent in accordance with priorly known techniques, also tend to decompose at the diffusion rate of oxygen (sometimes in a pyrophoric manner) thus giving rise to a constant concern of oxygen contamination.
It is also known that thin-film magnets produced in the presence of a solvent are very prone to having undesirably modulated magnetic properties due to coordination to the metal ion thereby blocking spin-coupling pathways and creating structural disorder and thus suppressing the film""s magnetic ordering temperature, Tc. Additionally, solvent synthesized magnetic films tend to be pyrophoric.
A low temperature (25-90xc2x0 C.) CVD based process in accordance with the present invention enables the fabrication of thin-film magnets on a variety of rigid and flexible substrates in the absence of a solvent such as acetonitrile, C2H3N, tetrahydrofiran, C4H8O, or dichloromethane, CH2Cl2, etc. Thin-film polymeric magnets of the present invention have enhanced air stability as well as other more favorable characteristics with respect to related magnets prepared from solution and containing solvent. Thus, the present invention provides the opportunity to prepare a broad range of magnetic structures, e.g., patterned and multilayered. Representative substrates in which the thin-film polymeric magnets of the present invention may be disposed upon include, but are not limited to, substrates comprised of alkali halides, e.g., sodium chloride (NaCl), potassium bromide (KBr) and cesium iodide (CsI), plastics/polymers including polytetrafluorethylene (PTFE), polyethylene teraphthalate (PET), and nylons, various crystalline and non-crystalline materials such as glasses, glass-ceramics, and ceramics, mica, semiconductive materials including gallium arsenide, silicon, and geranium, as well as metals such as aluminum, nickel, and alloys comprising aluminum-nickel including alnico magnets to name just a few of suitable materials. The thickness of the film may range to at least approximately 25 microns (25 xcexcm) with thicknesses ranging from approximately 20 nanometers (20 nm) to approximately 10 microns (10 xcexcm) being especially suitable for many applications.
Thin-film polymeric magnets embodying compositions and synthesized in accordance with the present invention are essential for many applications and the diverse new class of organic magnets are ideally suited for their development due to magnetic ordering temperatures, Tc, well above room temperature, modulation/tuning of properties via organic chemistry methodologies, compatibility with polymers for composites, mechanical flexibility, transparency, low temperature processibility, insulating to semiconducting behavior, solubility, etc. The present invention includes a method of synthesizing thin-film magnets composed of V[TCNE]x by a gas-phase chemical vapor deposition (CVD) process that is conducted at relatively low temperatures ranging from 25-90xc2x0 C. and at pressures ranging from 1-50 Torr, or greater. With respect to suitable gases to use in the CVD process, argon is particularly suitable and is constantly provided at rate to maintain the selected process pressure. However, nitrogen, helium, and carbon dioxide may be used in the CVD process as well as any of the chemically inert noble gases. During the CVD process, a flow rate of TCNE through an exemplary CVD apparatus, as disclosed herein, ranges from approximately 100 to 350 standard cubic centimeter per minute (100-350 sccm/min) with approximately 250 sccm/min being generally suitable for process pressures levels in the 1-50 Torr. The ratio of TCNE gas to V(CO)6 gas preferably ranges from 3:1 to 2:1. The CVD process is continued until the depth of the film on the selected substrate has been obtained. Whereupon the process is discontinued and the newly formed thin-film magnet, disposed on a selected substrate, is removed from the CVD apparatus. The thin-film magnet may then optionally be annealed at an elevated temperature (T greater than 350K) for a selected amount of time to alter the magnetic characteristics of the magnet if desired.
Alternately, rather than use TCNE, TCNQ may be used.
Optionally, bis(benzene)vanadium, V(C6H6)2, may be used as the vanadium containing process gas in lieu of V(CO)6 if desired.
Furthermore a suitable compositional make up of the magnetic thin film V[TCNE]x, where x=1.35 to 2, of the present invention is disclosed and is further defined and made identifiable in terms of magnetic, electrical, optical, and other defining characteristics. Such other defining characteristics include saturation magnetization properties ranging from approximately 10 emuOe/g to approximately 30 emuOe/g with electrical conductivity ranging from approximately 10xe2x88x925 S/cm to approximately 10xe2x88x924 S/cm.